The present invention relates to powder coating compositions; more particularly, the present invention relates to low temperature cure thermosetting powder coating compositions. The compositions consistently produce coatings having desirable performance properties when cured, and that are stable when uncured.
Coating compositions have long been used to provide the surface of articles with certain desired physical characteristics, such as color, gloss and durability. Many coating compositions rely on a liquid carrier, which evaporates after the composition is applied. In recent years, powder coatings have become increasingly popular; because these coatings are inherently low in volatile organic content (VOCs), their use reduces air emissions during the application and curing processes as compared with liquid coatings.
Powder coatings are typically cured by heating the coated substrate to an elevated temperature. These temperatures almost always exceed 125xc2x0 C., and commonly reach about 190xc2x0 C. to 205xc2x0 C. During the curing process, the powder particles melt and spread, and the components of the powder coating react. In addition to not emitting any VOCs into the environment during the application or curing processes, powder coating systems are extremely efficient since there is essentially no waste (i.e., application yield is approximately 100 percent). Because of the relatively high (i.e., greater than 125xc2x0 C.) cure temperatures of most powder coatings, their use, for practical purposes, is often limited to substrates that can withstand such high temperatures or that can be heated to an appropriate temperature long enough for cure to occur.
Despite the desirability of low-cure powder compositions, two problems have prevented their widespread production and usexe2x80x94their mechanical stability and their chemical stability. Powders that use resins with a glass transition temperature (xe2x80x9cTgxe2x80x9d) lower than 60xc2x0 C. usually encounter package stability problems, especially if exposed to prolonged heat exposure, and become fused, sintered or clumpy within days. Similarly, prolonged heat exposure can destroy the chemical stability of a powder if it includes crosslinkers that react at temperatures below about 170xc2x0 C.; if a crosslinker with a lower cure temperature is used, cure may be initiated during storage even though the film has not been formed. The premature gelation that occurs in these powder formulations results in coatings having shortened gel times. It is not unusual for low-cure powders to lose  greater than 50 percent of their gel time as a result of the premature gelation.
Problems encountered when a powder loses either mechanical or chemical stability can be severe. Poor mechanical stability creates obvious handling, application and appearance issues. Poor chemical stability creates subtler yet just as problematic issues. For example, a powder that has poor chemical stability will fluidize and apply like virgin powder, but because it has advanced in reactivity (i.e. undergone some premature gelation), it demonstrates restricted flow or no flow at all during cure. The result can be a coating having an xe2x80x9corange peelxe2x80x9d appearance, a rough texture or gel bodies.
Ideally, a powder should not lose its handling properties under elevated temperature storage and the gel time should remain the same as that of the virgin material. To achieve this, powders are typically formulated with resins having a Tg greater than about 60xc2x0 C. and/or crosslinkers that react at temperatures of about 170xc2x0 C. or greater. Such powders, however, are not low cure. Low-cure powders having lower Tg resins or lower temperature crosslinkers can require expensive storage under refrigeration and air-conditioned application facilities to overcome inherent lack of stability, or must be prepared using special techniques.
Thus, there is a need in the coatings art for low-cure powder coatings having a wide range of application, which also have an acceptable level of durability when cured on the finished product and good stability at room temperature.
The present invention is directed to powder coating compositions generally comprising a tertiary aminourea compound, a tertiary aminourethane compound, or mixtures thereof, and a film-forming polyepoxide resin. It has been surprisingly discovered that polyepoxide resins, when used with the present tertiary aminourea and/or aminourethane compositions, cure to form a suitable coating without the aid of crosslinkers, accelerators, or other additives typically regarded in the art as being necessary to cure a polyepoxide resin. The cured coatings that result from the present compositions have performance properties that are at least as good as powder coating compositions prepared with the same polyepoxides and conventional curing agents, but lacking the tertiary aminourea or aminourethane compositions described herein. Significantly, this desirable result is achieved by using curing temperatures much lower than those used for conventional products. Accordingly, the present compositions are low-cure. xe2x80x9cLow-curexe2x80x9d as used herein refers to powder coating compositions that cure at a temperature between about 80xc2x0 C. and 125xc2x0 C. However, the present invention is not limited to this temperature range and also provides cured films at temperatures up to and even greater than 190xc2x0 C.
As a result of being low-cure, the present compositions can be used on substrates that are not appropriately exposed to temperatures greater than about 125xc2x0 C. Examples include, but are not limited to, plastics such as thermoset and thermoplastic compositions, wood, and pieces of thick metal that cannot be heated above about 95xc2x0 C. because of their size. Also suitable are articles of manufacture that include a variety of substrates; for example, motors that contain both metal and rubber components can be suitably coated using the present, low-cure powder compositions.
The present compositions also overcome some of the difficulties that have been observed with other powder coating compositions, particularly other low-cure powders. For example, the present powder compositions are storage stable, and reduce, if not eliminate, the problems with chemical and mechanical stability seen with other low-cure powder compositions. The present compositions can be stored at room temperature, and they do not continue to catalyze the reaction of the polyepoxide molecules after the removal of heat. Moreover, the present powder compositions can be prepared using standard methods known in the art for preparing powder coating compositions; no special processing or handling is needed. Thus, the present compositions provide a significant advance in the low-cure powder coatings art.
Methods for coating substrates using the present powder compositions, and substrates coated thereby, are also within the scope of the present invention. Various low-cure catalysts are also included in the present invention.
The present invention is directed to a powder coating composition comprising: (a) a material having the structure of Formula I: 
wherein R1 is an organic radical having 6 to 25 carbon atoms, R2 is an organic radical having 1 to 20 carbon atoms; R3 and R4 are independently alkyl or phenyl groups having 1 to 8 carbon atoms; Z is oxygen or nitrogen and when Z is oxygen R5 is absent and when Z is nitrogen R5 is hydrogen or 
and n is 1 to 4; and (b) a polyepoxide. It will be understood that when Z is oxygen a tertiary aminourethane compound is represented and when Z is nitrogen, Formula I depicts a tertiary aminourea compound. It will be further understood that when R5 is 
there will be two each of R2, R3 and R4. Each R2, each R3 and each R4 can be the same or different as the other R2, R3 or R4. For example, one R2 can have one carbon and the other have two carbons, and the like.
The material of Formula I can be an oligomer wherein R1 is a monovalent, divalent, trivalent or tetravalent organic radical; divalent radicals are particularly suitable. The R1 radical can be aliphatic, such as hexamethylene, cycloaliphatic such as cyclohexylene, substituted cycloaliphatic such as 1,1,3,3-tetramethylcyclohexylene, or aromatic such as phenylene. Substituted cycloaliphatics are particularly suitable, especially 1,1,3,3-tetramethylcyclohexylene. Examples of suitable R2 moieties include ethylene, n-propylene, and iso- and n-butylene. In a particularly suitable composition, Z is nitrogen, R1 is 1,1,3,3-tetramethylcyclohexylene, R2 is propylene, R3 and R4 are both methyl groups, R5 is hydrogen and n is 2.
The material of component (a) can be prepared by reacting an organic polyisocyanate, particularly diisocyanate, with an amine containing a primary or secondary amine group and a tertiary amine group for the aminourea embodiment or with an alcohol or polyol containing a tertiary amine for the aminourethane embodiment. Suitable polyisocyanates include aliphatic, cycloaliphatic, or aromatic polyisocyanates. Diisocyanates are particularly suitable, although higher polyisocyanates can be used. Examples of suitable aromatic diisocyanates are 4,4xe2x80x2-diphenylmethane diisocyanate, 1,3-bis(1-isocyanato-1-methylethyl)benzene and derivatives thereof, and toluene diisocyanate. Examples of suitable aliphatic diisocyanates are straight chain aliphatic diisocyanates such as 1,6-hexamethylene diisocyanate and cycloaliphatic diisocyanates including isophorone diisocyanate and 4,4xe2x80x2-methylene-bis-(cyclohexyl isocyanate). Examples of suitable higher polyisocyanates are 1,2,4-benzene triisocyanate, polymethylene polyphenyl isocyanate and the isocyanurate of isophorone diisocyanate. Isophorone diisocyanate is especially suitable.
Examples of amines containing a primary or secondary amine group and a tertiary amine group are dimethylaminopropylamine, bis(dimethylamino)propylamine and 2-amino-5-diethylaminopentane. An example of an alcohol containing a tertiary amine is dimethylaminopropanol. Dimethylaminopropylamine is particularly suitable.
The diamine or amino alcohol and polyisocyanate are combined in an equivalent ratio of about 1:1. The diamine is heated to about 50xc2x0 C., and the polyisocyanate is added over a period of time in the range of about one to two hours, usually about two hours. The amino alcohol typically should be heated to about 80xc2x0 C. before the polyisocyanate is added. The temperature of the reaction mixture generally increases and is held at an elevated temperature, such as 130xc2x0 C. to 170xc2x0 C., until the polyisocyanate is completely reacted.
The present invention is further directed to a curable powder composition comprising a polyepoxide and the reaction product of a polyisocyanate and either an amine comprising a primary or secondary amine group and a tertiary amine, or an alcohol or polyol containing a tertiary amine. Suitable amines and alcohols/polyols, and the method for preparing such a reaction product, are described above.
In one embodiment, the material of component (a) further comprises an acidic hydrogen-containing compound; for example, component (a) can comprise the reaction product of (i) a compound having Formula I and (ii) an acidic hydrogen-containing compound. The acidic hydrogen-containing compound of (ii) may be a carboxylic acid, a phenolic compound, a polyester, a polyurethane or an acrylic polymer. Phenolic compounds, especially polyphenols, are particularly suitable. Examples of suitable acidic hydrogen-containing compounds include benzoic acid, dodecanedioic acid, azelaic acid, itaconic acid, sebacic acid, and adipic acid. Suitable phenols include phenol itself and polyphenols such as resorcinol, catechol, hydroquinone, bis(4-hydroxyphenyl)-2,2-propane (Bisphenol A), bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxyphenyl)-1,1-ethane, bis(2-hydroxyphenyl)-methane, 4,4-dihydroxybenzophenone, and 1,5-dihydroxynaphthalene. Bisphenol A is especially suitable.
The reaction product used in the coatings of the present invention can be prepared by mixing the material having Formula I of (i) with the acidic hydrogen-containing compound of (ii) in an equivalent ratio of about 1:1 to 1:2, such as about 1:1.87. The material of (i) is typically heated to a temperature of about 140 to 180xc2x0 C. and the acidic hydrogen-containing compound of (ii) is added. The reaction mixture is then usually held at the elevated temperature until it turns clear, indicating homogeneity of the reaction mixture. The reaction mixture is then allowed to cool.
Component (a) in the present compositions, both with and without the acidic hydrogen-containing compound, is used as a catalyst, and typically has a melting point of between about 23xc2x0 C. and 150xc2x0 C., such as between about 50 and 100C. This range of melting points helps prevent any curing from taking place in the composition before the application of heat. This improves the long-term stability of curable compositions in which component (a) is used. The melting point of the catalyst is typically not so high, however, that the present compositions lose their characterization as xe2x80x9clow-curexe2x80x9d. It is therefore desirable that the catalyst used in the present compositions have a melting point of between about 23xc2x0 C. and 150xc2x0 C.; if the melting point were too far above this number, the composition might not cure in the desired manner, and at temperatures too much below this temperature, the composition may not be as stable.
The polyepoxides used in the present compositions are those that are suitable for use in powder coatings, such as those that contain at least two 1,2-epoxide groups per molecule. In general, the epoxy equivalent weight can range from about 180 to about 4000 based on solids of the polyepoxide, such as between about 500 and 1000. The polyepoxides may be saturated or unsaturated, and may be aliphatic, alicyclic, aromatic, or heterocyclic. They may contain substituents such as halogens, hydroxyl groups, and ether groups.
Suitable classes of polyepoxides include epoxy ethers obtained by reacting an epihalohydrin such as epichlorohydrin with a polyphenol in the presence of an alkali. Suitable polyphenols include resorcinol, catechol, hydroquinone, bis(4-hydroxyphenyl)-2,2-propane (Bisphenol A), bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxyphenyl)-1,1-ethane, bis(2-hydroxyphenyl)-methane, 4,4-dihydroxybenzophenone, and 1,5-dihydroxynaphthalene. The diglycidyl ether of Bisphenol A is especially suitable.
Other suitable polyepoxides include polyglycidyl ethers of polyhydric alcohols. These compounds may be derived from polyhydric alcohols such as ethylene glycol, propylene glycol, butylene glycol, 1,6-hexylene glycol, neopentyl glycol, diethylene glycol, glycerol, trimethylol propane, and pentaerythritol. These compounds may also be derived from polymeric polyols such as polypropylene glycol.
Examples of other suitable polyepoxides include polyglycidyl esters of polycarboxylic acids. These compounds may be formed by reacting epichlorohydrin or another epoxy material with an aliphatic or aromatic polycarboxylic acid such as succinic acid, adipic acid, azelaic acid, sebacic acid, maleic acid, 2,6-naphthalene dicarboxylic acid, fumaric acid, phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, or trimellitic acid. Dimerized unsaturated fatty acids containing about 36 carbon atoms (Dimer Acid) and polymeric polycarboxylic acids such as carboxyl terminated acrylonitrile-butadiene rubber may also be used in the formation of these polyglycidyl esters of polycarboxylic acids.
Polyepoxides derived from the epoxidation of an olefinically unsaturated alicyclic compound are also suitable for use in the curable composition of the present invention. These polyepoxides are nonphenolic and are obtained by epoxidation of alicyclic olefins with, for example, oxygen, perbenzoic acid, acid-aldehyde monoperacetate, or peracetic acid. Such polyepoxides include the epoxy alicyclic ethers and esters well known in the art.
Other suitable polyepoxides include epoxy novolac resins. These resins are obtained by reacting an epihalohydrin with the condensation product of aldehyde and monohydric or polyhydric phenols. A typical example is the reaction product of epichlorohydrin with a phenol-formaldehyde condensate.
The curable composition of the present invention may contain one polyepoxide or mixtures of polyepoxides.
Typically, the polyepoxide is present in the curable composition of the present invention in a range of from about 20 to about 90 percent, such as about 30 to 60 percent, based upon total weight of the curable composition. The catalyst or reaction product is typically present in the compositions of the invention in a range of from about 0.5 to 10 weight percent, such as 3 to 5 weight percent. It is expected that the rate of cure increases as the concentration of catalyst increases, and that these increases are directly proportional. It is surprising, however, that no decrease in chemical or mechanical stability is noted as higher catalyst levels are used; stability often behaves inversely proportional to reactivity, in that as reactivity increases, stability decreases. This maintained stability with increased reactivity is yet another advantage of the present invention.
The powder coating compositions of the present invention may optionally contain additives such as waxes for flow and wetting, flow control agents, such as poly(2-ethylhexyl)acrylate, degassing additives such as benzoin and MicroWax C, adjuvant resin to modify and optimize coating properties, antioxidants and the like. These optional additives, when used, can be present in amounts up to 10 weight percent, based on total weight of the coating composition, and if used will typically comprise about 1 to 5 weight percent. Any of various pigments standardly used in the powder coatings art can also be included. Pigment weight can be up to 80 percent of the weight of the entire coating and usually is around 35 weight percent of the coating. The compositions can further comprise a plurality of particles, such as organic or inorganic particles, or mixtures thereof, that contribute to the mar and/or scratch resistance of the coatings. Such particles are described in Ser. No. 10/007,149, filed on Dec. 5, 2001, which is hereby incorporated by reference. Pigments or solid additives in nanoparticulate form can also be included in the present compositions for the same purpose.
It is both a significant and surprising discovery that the present compositions will cure at low temperatures in the absence of any additional components, such as a crosslinking agent and/or accelerator typically used in conjunction with polyepoxide resins, and thought to be required. In some cases, the use of a crosslinker and accelerator can actually raise the temperature required to cure the polyepoxide, so their use may be undesirable for a low-cure product. Although the inventors do not wish to be bound by any mechanism, it is believed that the reaction product or catalyst used in the present composition catalyzes the reaction of the polyepoxide molecules with themselves. This is in contrast to the standard mechanism of action, in which such a catalyst would be expected to facilitate the reaction between the polyepoxide and crosslinking agent. Thus, the present invention is further directed to a method for initiating self cure of a polyepoxide by adding any of the catalysts described herein to a composition comprising a polyepoxide.
Notwithstanding the lack of a crosslinking agent, the crosslinked density of the present coating compositions can still be controlled to a large extent. This is accomplished by controlling the amount of catalyst added to the composition. Higher amounts of catalyst usually gel the films faster and crosslink the films more efficiently. In addition, there is a cost savings associated with the elimination of crosslinkers and accelerators, and the ability to cure at a lower temperature. Significantly, the present crosslinker-free and accelerator-free compositions result, upon curing, in coating compositions that have performance properties at least equal to that of conventional powder coatings in which a polyepoxide and conventional crosslinker are used. This refers to the ability to maintain appearance as measured by a number of properties relevant to cured coatings, such as resistance to solvents, pencil hardness, and impact and corrosion resistance.
The present compositions can be prepared by standard methods known in the art. For example, the components are first thoroughly mixed to ensure spatial homogeneity of the ingredients. The composition is then intimately melt kneaded in an extruder. Typical zone temperatures during extrusion range from 40xc2x0 C. to 125xc2x0 C., such as 45xc2x0 C. to 100xc2x0 C. The exiting extrudate is rapidly cooled to terminate polymerization. The resulting chip is then micronized into powder with an average particle size of 0.1 to 200 microns, such as 1 to 100 microns. Comminution methods are well known, comminution can be accomplished, for example, by air-classifying mills, impact mills, ball mills, or other fracture-induced mechanisms. Post additives that improve fluidization of the powder mass and/or improve the resistance to impact fusion may be incorporated into the final product before or after micronization. As noted, the use of standard powder coating preparation methods is another advantage of the present invention.
Accordingly, the present invention is further directed to powder coating compositions that cure at a temperature of between 80xc2x0 C. and 125xc2x0 C. comprising a resin and curing agent and wherein substantially all of the curing agent is extruded with the resin. xe2x80x9cSubstantially allxe2x80x9d means the amount of curing agent needed to completely cure the resin. The present invention is further directed to such compositions that do not cure at temperatures below about 70xc2x0 C., such as at ambient temperature, like many commercially available low-cure products.
Typically, the present powder coatings will have average particle sizes that range between 15 and 200 microns, such as between about 25 and 50 microns.
The powder coating compositions of the present invention can be applied to a substrate in any number of ways, most often by electrostatic spraying. The powder coating can be applied in a single sweep or in several passes to provide a film having a thickness after cure of from about 1 to 10 mils (25 to 250 microns), usually about 2 to 4 mils (50 to 100 microns). Other standard methods for coating application can also be employed.
After application, the present compositions may be cured by heating to a temperature of between about 80xc2x0 C. and 190xc2x0 C., preferably between about 80xc2x0 C. and 125xc2x0 C., for a period ranging from about 3 minutes to 30 minutes, such as 15 to 20 minutes. Heating can be effected by any means known in the art, typically by placing the coated substrate in an oven. IR radiation can also be used to heat cure the coated substrates.
Accordingly, the present invention is further directed to a method for coating a substrate comprising applying to the substrate one or more of the coating compositions described herein and curing the coating at a temperature of between about 80xc2x0 C. and 190xc2x0 C., such as between about 80xc2x0 C. and 125xc2x0 C. or between about 105xc2x0 C. and 120xc2x0 C. In such a method, the polyepoxide will self-cure, or react with itself by homopolymerization; this reaction is catalyzed by the present tertiary aminourea or tertiary aminourethane compositions. Accordingly, the present invention is further directed to a cured coating layer comprising a polyepoxide and one or more of the catalysts described herein, wherein the polyepoxide is self-cured.
A number of substrates are suitable for coating according to the methods of the present invention, including plastics such as thermosets or thermoplasts, cardboard, paper, wood, metal, particleboard and medium density fiberboard or mixtures thereof. Substrates coated according to the present methods are also within the scope of the present invention.
The present invention is further directed to a catalyst composition that is the reaction product of:
(i) a material having the structure of Formula II and
(ii) an acidic hydrogen-containing compound 
For Formula II, R1 is an organic radical having 6 to 25 carbon atoms, R2 is an organic radical having 1 to 20 carbon atoms; R3 and R4 are independently alkyl or phenyl groups having 1 to 8 carbon atoms; Z is oxygen or nitrogen and when Z is oxygen R5 is absent and when Z is nitrogen R5 is hydrogen or 
and n is 1 to 4, but when Z is nitrogen, R2 is an alkylene having between 1 and 4 carbon atoms, and R3 and R4 are both alkyl groups having between 1 and 4 carbons, R5 is not hydrogen.
The present invention is directed to yet another catalyst composition comprising the compound of Formula I as described above, wherein the composition does not include an acidic hydrogen-containing compound. It has surprisingly discovered that compounds having Formula I function as low-cure catalysts even in the absence of acidic hydrogen-containing compounds.
As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word xe2x80x9caboutxe2x80x9d, even if the term does not expressly appear. Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein. As used herein, the term xe2x80x9cpolymerxe2x80x9d refers to oligomers and both homopolymers and copolymers; the prefix xe2x80x9cpolyxe2x80x9d refers to two or more.